In SiC (silicon carbide), a high-temperature type (α-type) having a hexagonal crystal structure and a low-temperature type (β-type) having a cubic crystal structure are known. SiC is characterized, in comparison with Si, by having a high thermal resistance, a broadband gap, and a high dielectric breakdown electric field strength. For that reason, a semiconductor including an SiC single crystal is expected as a candidate material of a next-generation power device substituting for an Si semiconductor. In particular, α-type SiC has a band gap broader than β-type SiC and hence the α-type SiC attracts attention as a semiconductor material of an ultralow power-loss power device.
The α-type SiC has, as the principal crystal plane, a {0001} plane (to be also referred to as “c-plane” hereunder), a {1-100} plane (m-plane), and a {11-20} plane (a-plane in a narrow sense) the latter two of which are perpendicular to the {0001} plane. In the present invention, “a-plane” means a-plane in a broad sense, that is, “generic name for m-plane and a-plane in a narrow sense”. When it is necessary to distinguish the m-plane from the a-plane in a narrow sense, they are referred to as “{1-100} plane” and “{11-20} plane”, respectively.
A c-plane growth method has heretofore been known as a method of obtaining an α-type SiC single crystal. The “c-plane growth method” cited here means a method of using as a seed crystal an SiC single crystal in which a c-plane or a plane having an offset angle to the c-plane in a prescribed range is exposed as a growth plane and growing an SiC single crystal over the growth plane by a sublimation reprecipitation method or the like.
The problem has, however, been that, in a single crystal obtained by the c-plane growth method, a large number of defects such as micro pipe defects (tubular voids about several μm to 100 μmin diameter) and threading screw dislocation (hereunder referred to also merely as “screw dislocation”) are generated in the direction parallel to the <0001> direction. To realize a high-performance SiC power device, the reduction of a leak current generated in an SiC semiconductor is essential. It is considered that defects such as micropipe defects and screw dislocation which are generated in the SiC single crystal cause an increase in this leak current.
Then, to solve this problem, various proposals have been made.
For example, Patent Document 1 discloses a growth method of an SiC single crystal for growing an SiC single crystal by using a seed crystal having a plane (for example, {1-100} plane or {11-20} plane) which is inclined at about 60° to about 120° from the c-plane and exposed as the growth plane (this growth method will be referred to as “a-plane growth method” hereinafter).
The above document teaches that:    (1) when SiC is grown on a crystal plane inclined at about 60° to 120° from the c-plane, an array of atomic layers appears on the crystal plane, thereby making it easy to grow a crystal having the same type of a polytype structure as that of the original seed crystal;    (2) when this method is employed, screw dislocation does not occur; and    (3) when the seed crystal includes dislocation having a slip plane on the c-plane, this dislocation is carried over to the growing crystal.
Patent Document 2 discloses a growth method of an SiC single crystal, including the steps of:
growing SiC by using a seed crystal having a {10-10} plane as the growth plane;
taking out a {0001} wafer from the obtained single crystal; and
growing SiC by using this wafer as a seed crystal.
The above document teaches that, by this method,    (1) an SiC single crystal having few micropipe defects is obtained; and    (2) since a much larger {0001} wafer than an Acheson crystal is obtained, it is used as a seed crystal to grow a large-sized crystal easily.
Patent Documents 3 and 4 disclose a method for producing an SiC single crystal, including the steps of:
carrying out a-plane growth in orthogonal directions a plurality of times; and
carrying out c-plane growth in the end.
The above documents teach that:    (1) as the number of repetitions of a-plane growth increases, the dislocation density in the growing crystal decreases in an exponential fashion;    (2) the generation of a stacking fault during a-plane growth cannot be avoided; and    (3) when c-plane growth is carried out in the end, micropipe defects and screw dislocation do not occur and an SiC single crystal having almost no stacking fault is obtained.
Further, Patent Document 5 discloses a growth method of an SiC single crystal in which a seed crystal with a crystal plane having an offset angle of 5° to 30° in the (000-1) C-plane direction from a plane perpendicular to the (0001) basal plane of an SiC single crystal is used in the sublimation reprecipitation method.
The above document teaches that:    (1) when a plane offset to the (000-1) C-plane direction is used, crystal grains having different crystal orientation is hardly obtained as compared with when a plane offset to the (0001) Si-plane direction is used;    (2) when a plane offset to the (000-1) C-plane direction is used, such grains are rarely produced as compared with a nonpolar plane perpendicular to the (0001) basal plane; and    (3) when the offset angle to the (000-1) C-plane direction is too large, the mixture of polytype readily occurs and a large number of crystal defects are produced.
The a-plane growth method has an advantage that an SiC single crystal having a low screw dislocation density is obtained. However, the a-plane growth method has a problem that high-density stacking faults almost parallel to the c-plane tend to be produced. When a stacking fault occurs in the SiC single crystal, electric resistance in a direction across the stacking fault increases. Therefore, an SiC single crystal having such stacking faults at a high density cannot be used as a semiconductor for power devices.
Meanwhile, it is considered that when c-plane growth is carried out after a-plane growth is carried out at least one time, an SiC single crystal having almost no screw dislocation and no stacking faults can be produced.
To manufacture a substrate having a large diameter of the {0001} plane by using a method making use of a combination of a-plane growth and c-plane growth, a crystal which is prolonged in both {11-20} plane growth direction and {1-100} plane growth direction which is inclined at 90° from that plane must be grown. However, in the {11-20} plane growth and the {1-100} plane growth, differently oriented crystals or heterogeneous polytype crystals are often produced on the growth plane.
To solve this problem, as disclosed by Patent Document 5, it is proposed to use a crystal plane having an offset angle of 5° to 30° in the (000-1) C-plane direction from a plane perpendicular to the basal plane. However, when the offset angle is small, it is impossible to completely suppress the production of differently oriented crystals. When the offset angle is large, a heterogeneous polytype layer which grows outward from the end on the C-plane side of a seed crystal expands into the growing crystal along the growth of the crystal. Therefore, the final yield when the {0001} plane substrate is cut out is reduced, and it is difficult to cut out a large-diameter substrate.